is a "boutique" tube screamer, if you will. More bass and more available gain range (.1uf / 3.3k, 1 meg gain pot w/ 10K series). Mid-hump is softened resulting in a more up-front sound. The clipping diodes include a 1N270 Germanium to make the clipping slightly asymmetrical.
The circuit mods are the same as other successful TS clones and common upgrades.
If you like the sound of other "boutique" TS clones, and want that sound in your ZIM, B9 is the way to go. With the voicing section in Ch B, and the ability to be driven harder by Ch A, B9 will give you what you always wanted from that boutique overdrive.